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What IS arc flash and arc blast?
For many years the hazard of electrical shock has been well understood. The National Electric Code and OSHA regulations provided consumers and workers protection from shock hazard. Increasingly, it became recognized that a large percentage of electrical injuries were burns not technically related to electrical shock-that is contact with a live electrical conductor. As this phenomenon was investigated, it became clear that there was another electrical hazard to considered.
Arc flash occurs when there is an arcing fault between two energized conductors or between and energized conductors and ground. The arc begins when there is breakdown in the insulation of the conducting elements. It may be precipitated by a short-circuit or degradation of the insulation separating the conductors. The arc can grow massively depending upon the amount of current available in the circuit and the amount of time it takes for a fuse or breaker to clear the fault. As the arc begins it produces a cloud around it of electrically conductive plasma, which allows even more current to flow into the arc. In a fraction of a second, the arc can produce prodigious amounts of light and heat enough to directly burn skin and set afire clothing.
Arc blast is the extreme form of arc flash as the heat builds it causes explosive expansion of the super-heated air and vaporizing metals in the vicinity of the arc. It is estimated that an arc flash/arc blast can produce heat as high as 35,000° and a blast pressure wave equivalent to several sticks of dynamite propelling molten shrapnel at 700 mph.
Why should we do an arc flash hazard analysis study?
There are three associated answers. First, safety is simply the right thing to do. A responsible company will want to protect its workers from a recognized hazard. The second reason is economical considerations. Failing to remedy dangerous conditions is like a ticking time bomb or a risky gamble. Should an arc flash event occur it is likely that there will be hefty associated costs paid in downtime, equipment repair, not to mention medical care for injured employees. Thirdly, a proactive approach to remedying arc flash hazards insures that your company remains in compliance with OSHA regulations and the NFPA 70E.
Specifically, section 130.3 of the NFPA 70E-2009 says,
An arc flash hazard analysis shall determine the Arc Flash Protection Boundary and the personal protective equipment that people within the Arch Flash Protection Boundary shall use.
The arc flash hazard analysis shall be updated when a major modification or renovation takes place. It shall be reviewed periodically, not to exceed five years, to account for changes in the electrical distribution system that could affect the results of the arc flash hazard analysis.
The arc flash hazard analysis shall take into consideration the design of the overcurrent protective device and its opening time, including its condition of maintenance.
If an arc flash hazard analysis has never been done upon your company's electrical equipment, one needs to be accomplished ASAP to remain in compliance with government regulations.
Please describe the arc flash hazard analysis process?
Arc flash hazard analysis is by nature a very technical and somewhat lengthy process. It is possible that a company may have the wherewithal to conduct the arc flash study, but usually the company contracts with an electrical engineering firm with experience in the field to conduct the study. The arc flash hazard analysis study should be viewed as a cooperative partnership between the client company and the contracted engineers to facilitate the study and to reap the greatest possible gains.
In general there are seven phases in the analysis:
Phase 1: Data Collection
Accurate data is absolutely necessary to produce useful results. In order to perform the study engineering technicians will need to collect detailed nameplate information during visit to your facility. They will also need access to all of the breakers, fuses, panels and control panels. They will collect the data utilizing NFPA 70E standards for PPE while opening the cabinets to obtain the information. Any assistance the client company can provide in the areas of any available drawings, nameplate data, manuals or other information related to the electrical equipment will streamline this process.
It is possible that the data collection phase can be accomplished by the client company's employees, however the potential for missing or inaccurate data may markedly slow the progress of the study.
Data Collection Note:
Along with the data collection process, the engineering technicians will conduct a safety survey and report to you any safety and compliance issues they may note.
Phase 2: Single-Line Diagram (SLD) and Computer Model
The engineering technicians will assess the electrical hazards by building an accurate single-line-diagram of the facility and converting them intotheir digitial equivalents. This single line model is used to calculate the short-circuit currents.
The completed Single Line Diagram is required for a qualified professional engineer to calculate the fault currents at these points. The complete single-line diagrams will become part of the final report submitted to client and should be updated regularly by the company
Phase 3: Short-Circuit (SC) Study
Determining the short-circuit current for each bus and branch is one of the most important aspects of designing and maintaining your power distribution system and is a key component in the process of calculating arc flash potential. Short-circuits must be anticipated and their effects considered when selecting electrical equipment. Inadequate devices represent possible failure with damage, injury, and repair expense. The data obtained from fault calculations determines the sizing of switchgear and all related protective equipment. The results of this phase of the study will be based on applicable ANSI/IEEE standards.
- Maximum RMS symmetrical three-phase bus short-circuit current available at each substation and each motor control center. The results will represent the highest short-circuit currents to which feeder/load equipment might be subjected under the reported system conditions. Estimates for contributions from large induction motors will be included in the calculations.
- Comprehensive list of the input data and the calculated results.
- A one-line diagram (specific to the computer model) will identify all bus locations with the maximum available bus short-circuit current. The maximum branch short-circuit current will be shown for each protective device.
- Evaluation of the adequacy of the short-circuit ratings of the protective devices will be provided in tabular form. Any inadequacies will be called to your attention, and recommendations will be made for improvement.
Phase 4: Protective Device Coordination (PDC) Study
The time it takes for a given protective device to trip during a fault is a major contributor to the magnitude of the potential arc flash. Consequently, examination of the protective devices is necessary to identify how they are used to isolate equipment faults. The tripping characteristics (the time-current characteristics curves -- TCC) for breakers and fuses are studied to see if they are properly set according to three limits. First, they are set below a maximum level to protect the downstream equipment. Second, they are set above a minimum level to prevent nuisance tripping during startup and normal operation. Third, they are set below the upstream device. Devices set within these constraints are said to be "selective", or well-coordinated.
The purpose for performing the Coordination Study is to examine the timing of protective devices to insure that such devices are properly coordinated in the event of a fault in the system. This is accomplished by comparing the Time Current Curves (TCC) of the related protective devices based upon the manufacturer's published data and the device settings. In the event any miscoordination is discovered, the study will provide recommendations for better coordination of the protective device.
Phase 5: Arc-Flash Hazard (AFH) Analysis
This analysis builds upon the existing short-circuit analysis, and protective device coordination.
A key point contained in this IEEE 1584 standard pertaining to the calculation of arc-flash worst case values is that small reductions in arcing fault current can produce substantially different protection device trip times and thus higher arc-flash levels. Therefore, the standard recommends that low tolerance of 15% for arcing fault current calculations be used. Thus, the arc-flash should be calculated at the low (85%) and high (100%) tolerance values utilizing the highest arc-flash result for determining proper NFPA 70E arc-flash protection.
A key point to remember is that optimal arc-flash protection could still lead to a second-degree skin burn, which is considered curable and therefore tolerable.
Arc blast hazards exist when the energy released from and arc flash exceeds 40 cal/cm2 at ordinary working distance. In the case of arc-blast, there are few personal protection options. This hazard consists of the molten metal debris and shrapnel, and of the exploding metal vapors. If the available energy from an arcing-fault is too large for arc-flash protective clothing, then the only other measures are distance and barriers. As with arc-flash, the energy can be reduced in large part by limiting the available arcing-current, or by reducing the arcing-time with faster breakers and fuses.
Phase 6: Electrical Preventive/Predictive Maintenance Program
According to NFPA 70E-2009 section 210.5 FPN "Failure to properly maintain protective devices can have an adverse effect on the arc flash hazard analysis incident energy valuses." As a part of the system study and arc flash evaluation the engineering technicians will provide a written electrical preventive/predictive maintenance program for all electrical devices and equipment contained in the scope of the system study and arc flash. The written EPPM program provided is based on: NFPA 70E, Chapter 2 Safety Related Maintenance Requirements", NFPA 70B "Recommended Practice for Electrical Equipment Maintenance" and NETA.
Implementing and maintaining an electrical preventive/predictive maintenance program will provide two benefits. First, EPPM will provide for increased reliability of the plant electrical system and secondly it will meet the requirements of the NFPA 70E arc flash program.
All arc flash calculations are based on the arc clearing time and short circuit current in order to determine incident energy. With this information protection boundaries are determined. The clearing time is derived from the protective device coordination study, mentioned in this proposal., which is based on manufacturer time current curves. If maintenance and testing is not performed it could result in extended clearing times, unintentional time delays, open or shunted current transformers, open coils, or dirty contacts. This can affect the calculated arc flash boundaries and the PPE requirements.
Phase 7: Label Creation and Installation
Clearly marking the danger level of each device is an essential stage in Arc Flash awareness. High Quality UV resistant labels providing clear warning of the Arc Flash Hazard and identifying the Personal Protective Equipment required will be produced and affixed to each device incorporated into the study. In most situation two varieties of-colored labels will be provided. In the case of devices that are in one of the four categories of arc flash hazards, orange and black warning labels will identify the hazards. For equipment that present arc blast hazards which may only be worked upon in the denergized state, red and white DANGER labels will be provided.
If we do an arc flash hazard analysis, what is our part in the process?
The most important thing you can do at the outset is provide accurate equipment data to the arc flash team; specifically an up-to-date single line drawing will materially speed up the arc flash hazard analysis.
Should you gather all the data yourselves? Some companies do conduct their own data collection in order to cut costs. Certainly, it is possible to gather your own data, but it is not always an ideal solution. The engineering firm's data collection team is experienced in knowing exactly what kind of data is needed. It is imperative that the data be as accurate as possible before the actual analysis. Should you opt to collect the data yourselves, the engineering firm will supply you with documentation specifying the needed data standards and forms to record the data.